Homework Help:
Kapteyn's and Shapley's Milky Way - a comparison

Hi all, new here and was wondering if you could help me out. I'm basically giving the two Milky Way models, and what was a common mistake (note it's a written piece not workings out) and I'd like to know if you think it makes sense/I don't waffle too much.

Thanks in advance

The attempt at a solution

Kapteyn mapped his version of the Milky Way Galaxy using the distances of stars, using only apparent brightness for the stars too far away to have a measurable proper motion (he did however estimate the size of the galaxy using stars of known parallax and proper motion). His galaxy consisted of a disk of stars with a diameter 5.1 times greater than its thickness. With a diameter of 55,000 light-years, a thickness of 11,000 light-years and a total of 47.4 billion stars this was quite a small Milky Way. More importantly though, Kapteyn placed the Sun near the centre of the galaxy “lying between 2000 and 2300 light-years from it” which was very flawed.

Shapley however focused on globular clusters to measure the size of the Milky Way. In particular he looked at Cepheids which are yellow supergiants with set and reliable pulsation periods. He used these yellow supergiants in particular because they obey a relationship in which the longer the pulsation period, the larger the star therefore the greater its mean intrinsic brightness. With this knowledge and the ease of measuring apparent brightness he could easily find the distances to the centre of the globular clusters. Shapley's Milky Way was much bigger than Kapteyn's at 330,000 light-years in diameter. Realising globular clusters would form near the galactic centre he also placed our Sun much further away at 65,000 light-years, and even though the figures are incorrect (Shapley's model is agreed to be about two times too bigger than the accepted modern model) he was right in thinking the Sun was not near the centre of the Milky Way. One cause of the bloated figures is that what Shapley thought were Cepheids were in fact W Virginis stars, which are intrinsically fainter giving the illusion that they were further away than thought.

There was however something that neither astronomer took into account when mapping their galaxies, and that was interstellar dust. This meant that some of the light from distant stars was absorbed and so they would always appear dimmer to the observer, making them seem fainter and further away. Kapteyn even looked into interstellar dust and found evidence for its existence, but later dismissed the idea and he remained confident in his galactic model. Unfortunately for Kapteyn, who was mapping the galaxy using stars alone, the dust had a much bigger impact on his model. The dust was blocking the view to the true centre of the galaxy, which is why he was led to believe the Sun was so close to it. Shapley used clusters which are much more luminous than lone stars so could be seen at greater distances more clearly – also the majority of them are found outside the plane of the Milky Way, which is where the gas and dust is at its thickest. That combined with the use of pulsating variable stars to measure distances saw a more realistic model of the Milky Way.